(1) Field of the Invention
The present invention relates to a display device using liquid crystals, especially to a liquid crystal display device and a display apparatus for being used in office information apparatus (OA apparatus) etc.
(2) Description of the Prior Art
Many kinds of liquid crystal display devices have been proposed and come to practical use as display devices for information display apparatus. At the present, a type using nematic liquid crystals represented by TN mode (Twisted Nematic; Refer to JP-A-47-11737) and STN mode (Super Twisted Nematic; Refer to JP-A-60-107020) is the main current and is widely used.
The TN mode and the STN mode have initial structural condition in which each of oriented direction of liquid crystal molecules is twisted approximately 90.degree. or 260.degree. in the cell. Incident light to the cell outgoes after being changed in polarized condition by the twisted structure of the liquid crystals and birefringence. When electric field is applied to the liquid crystal layer, the twisted structure vanishes by rearrangement of the liquid crystal molecules in direction of the electric field, and the outgoes without any change in the polarized condition. By composing a structure in which a liquid crystal cell is held between two linear polarizers, above described change (electro-optical effect) in optical property of the liquid crystal layer by application of the electrical field can be observed as change of outgoing light intensity.
With TN mode and STN mode, display contrast is obtained by the operation based on the above described principle.
The above described display device has such advantages as remarkably smaller consuming electric power than CRT (Cathode Ray Tube) display and making it possible to realize thin display panel. Accordingly, the apparatus is used widely in office information apparatus such as personal computers and word processors etc.
However, the above described display apparatus is substantially a type provided with polarizers, and the incident light is not utilized effectively. Actually, there are many cases of display in which a light source (a back light) is provided behind the liquid crystal display device for keeping brightness. A type provided with color filter (Color liquid crystal display) reduces quantity of transmitted light farther, and consequently, it needs stronger light source. Electric power consumed by the light source is equivalent to the consuming electric power of the liquid crystal panel including driving circuits, therefore, such type is not suitable for display of portable type information apparatus of which electric power is supplied from batteries. That is, brightness and lowering of consuming electric power in the conventional display device are in the so-called trade-off relation, and development of bright (high light transmittal) display device which does not need the back light is earnestly desired.
Fluorescent back light is not preferable in aspect of eye fatigue when watching the display continuously, and reflection type display is desired. Besides, the display device having high light transmittance contributes to reduction of light source size, extension of life, and electric power saving of the whole apparatus even in the case of being used as a projection type display.
In response to the above described need, a liquid crystal display device without using polarizers is proposed. The White-Taylor type guest-host cell [Refer to Journal of Applied Physics (J. Appl. Phys.) Volume 45, page 4718-4723 (1974)] is one of such display devices. Chiral nematic phase (cholesteric phase) liquid crystal is mixed with dichroic dye, and has a structure of approximate parallel configuration to the base plane. By applying electric field, the configuration of the liquid crystal changes, and consequent orientation change of the dichroic dye changes light transmittance. Owing to the twisted structure caused by the chiral nematic phase, light absorption by the dye is performed effectively, and theoretically high display contrast can be obtained without polarizers.
In the above described case, spiral pitch of the chiral nematic liquid crystal is necessarily same order of wave length of the light for realizing of high contrast. But, if the spiral pitch is shortened to that order, many disclination lines are generated and display quality becomes worse. At the same time, hysteresis occurs and response to the electric field becomes extremely slow. Accordingly, it is less practical in comparison with the TN mode and the STN mode.
Another representative display device without using polarizers is a display device called PDLC (Polymer Dispersed Liquid Crystal: Refer to JP-A-58-501631(1983)).
The above described device consists of polymer matrix in which nematic liquid crystal having positive dielectric anisotropy is dispersed as particles having diameter of a several micrometers. When refractive indexes of the liquid crystal to ordinary light and extraordinary light are represented by n.sub.o and n.sub.e respectively, and refractive index of the polymer is represented by n.sub.p, combination of liquid crystal and polymer material are so selected as to be n.sub.p .apprxeq.n.sub.o .noteq.n.sub.e. In the initial condition, the liquid crystals form a warped configuration structure in the particles, furthermore, difference of refractive index between most of the liquid crystal particles and the polymer matrix is induced by fluctuation in oriented direction among each of particles, consequently, the device scatters light as if it were frosted glass. When sufficient voltage is applied to the device, rearrangement of the liquid crystal molecules in the particles is induced and refractive indexes of the liquid crystal and polymer matrix to perpendicularly incident light become equal value. As the result, refraction and reflection at the boundary of the liquid crystal and polymer are almost disappeared, and the device changes to transparent state. The incident light is not necessarily linear polarized light.
As the displays based on the above described theory, polarizers are not necessary, and the incident light can be utilized effectively for obtaining bright display. But, in order to obtain sufficient display contrast, thick film having thickness of a several tens micrometers is necessary, consequently, its operating voltage becomes a several tens volts. Furthermore, as the device is dispersion type, it is preferable for the projection type display but not for the direct view type display such as OA apparatus etc.
Although a reflection type display, in which dichroic dye is mixed into nematic liquid crystal, contrived reflectors or cell construction is proposed (Refer to JP-A-59-178429 (1984) and JP-A-59-178428 (1984)), the improvement is not substantial, and it is hard to say sufficient.